High-conductivity high-purity zero-six-valence chromium sodium chromite, preparation method and use thereof

Abstract

This invention discloses a high-conductivity, high-purity sodium chromite with zero hexavalent chromium, its preparation method, and its applications. The method includes: a first step to achieve the high-purity zero hexavalent chromium properties of sodium chromite; and a second step to form a uniform carbon coating layer through vacuum carbon fixation with molten sugar. The advantages of this invention are: by synergistically optimizing the two-step process, the core problem of poor conductivity of pure-phase sodium chromite is solved, while avoiding the purity reduction caused by carbon coating. The resulting product combines high purity, zero hexavalent chromium residue, high conductivity, and good crystal integrity. Sodium-ion batteries assembled with this product exhibit high specific capacity, excellent rate performance, and cycle stability, showing broad application prospects.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode materials, and in particular to a high-conductivity, high-purity sodium chromite with zero hexavalent chromium, its preparation method, and its uses. Background Technology

[0002] Sodium chromite (NaCrO2), as a cathode material for sodium-ion batteries, has advantages such as stable layered crystal structure, excellent thermal stability, and high theoretical specific capacity. However, pure-phase sodium chromite has poor electronic conductivity (typically 10⁻⁻⁶). 6 ~10⁻ 4 The presence of hexavalent chromium (S / cm) limits the rate performance and cycle stability of the battery, restricting its application in high-performance sodium-ion batteries. Furthermore, the residue of hexavalent chromium not only reduces the electrochemical performance of the material but also poses serious environmental and safety hazards. Therefore, achieving synergistic optimization of high purity sodium chromite, zero hexavalent chromium residue, and high conductivity is currently a key technological challenge.

[0003] However, sodium chromite materials prepared using existing technologies still face two major technological bottlenecks in practical applications: First, the issues of purity and residual toxicity. Traditional synthesis of sodium chromite typically employs a high-temperature solid-state method using chromium oxide (Cr₂O₃), sodium carbonate (Na₂CO₃), and carbon powder. This method is problematic because the raw materials (especially industrial-grade chromium oxide) often contain trace amounts of hexavalent chromium (Cr₂O₃). 6 (⁺) Impurities and incomplete high-temperature reaction kinetics lead to the easy retention of trace amounts of hexavalent chromium in the final product. Hexavalent chromium is highly toxic and carcinogenic, limiting its application in food, medical, and consumer electronics fields, and also failing to comply with stringent environmental regulations such as the EU RoHS. Existing technologies, such as patent CN114195188A, mention improving purity through raw material pretreatment, but their impurity removal methods are limited and cannot completely block the formation pathway of hexavalent chromium or reduce and eliminate it, making it impossible to stably prepare a high-purity product with "zero hexavalent chromium". Second, there is the problem of low electronic conductivity. Sodium chromite itself is a semiconductor material with low intrinsic electronic conductivity (typically around 10⁻). 5 ~10⁻ 6(On the order of S / cm). While conductivity can be maintained at low charge-discharge rates, in high-power applications, low conductivity leads to severe polarization, resulting in capacity decay and shortened cycle life. To address this deficiency, existing technologies mainly improve conductivity through carbon coating or composite conductive agents. For example, patent CN115863609B discloses a method for achieving carbon coating through stepwise decarburization using a complexing agent. However, this method heavily relies on precise control of the complexing agent, resulting in a lengthy and complex process with high production costs. Furthermore, while some physical mixing methods are simple to operate, they can easily lead to uneven carbon layer coating or excessively high carbon content. Excessive inactive carbon can dilute the overall energy density of the material, resulting in capacity loss. Simply adding conductive agents (such as acetylene black or graphene) often fails to form a continuous and stable three-dimensional conductive network between particles.

[0004] In summary, current technologies lack a process for preparing sodium chromite that can simultaneously achieve both high purity (zero-hexavalent chromium) and high conductivity. In particular, how to utilize inexpensive and readily available carbon sources (such as sugars) to achieve uniform carbon fixation modification of the surface of high-purity sodium chromite through a simple process, thereby improving conductivity without introducing new impurities, is a pressing technical challenge in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a high-conductivity, high-purity sodium chromite with zero hexavalent chromium, its preparation method, and its uses, so as to at least partially solve the above-mentioned problems of the prior art.

[0006] To achieve the above objectives, this invention provides a method for preparing high-conductivity, high-purity sodium chromite with zero hexavalent chromium, characterized by comprising: Step 1, preparation of sodium chromite with zero hexavalent chromium: ① Raw material pretreatment: Chromium oxide is subjected to nitric acid-high pressure closed digestion pretreatment, soda ash is subjected to ultrasonic vibration recrystallization purification pretreatment, and carbon powder is subjected to high-temperature impurity removal pretreatment; ② Proportional mixing: The pretreated chromium oxide, soda ash, and carbon powder are mixed in a molar ratio of 1:(1.02~1.2):(0.01~0.05), and then pulverized by ultra-low temperature liquid nitrogen ball milling to obtain a uniform mixed powder; ③ Atmosphere-protected molding: The mixed powder is pressed into a tablet blank under an inert atmosphere to obtain a tablet blank; ④ Two-step sintering: In the first step, the tablet blank is placed in an atmosphere-protected electromagnetic rotary kiln for pre-sintering to achieve preliminary solid-phase reaction of the raw materials and removal of impurities; in the second step, it is transferred to an atmosphere-protected vacuum sintering furnace for final sintering to ensure the presence of zero hexavalent chromium; ⑤ Post-processing: The final sintered product is ball-milled in low-temperature liquid nitrogen, ultrasonically cleaned with polar solvent, and vacuum dried to obtain high-purity sodium hexavalent chromium chromite; Step 2, improving conductivity: ① Molten sugar mixing: The high-purity sodium hexavalent chromium chromite obtained in step (1) is mixed with molten sugar at a mass ratio of 100:(5~15) and stirred until a uniform coating slurry is formed; ② Vacuum carbonization sintering: The coating slurry is transferred to a vacuum sintering furnace under atmosphere protection and sintered and carbonized in three stages. The first stage is used to remove moisture and low-boiling-point impurities, the second stage is used to achieve carbonization and carbonization of sugar, and the third stage is cooled to obtain high-conductivity high-purity sodium hexavalent chromium chromite.

[0007] Further, in step one①, the chromium oxide is large-size single-crystal Cr2O3 with a particle size of 1~30μm; the nitric acid-high-pressure closed digestion pretreatment parameters are: chromium oxide to 5~20wt% nitric acid solution solid-liquid mass ratio 1:10~20, 5~10MPa, 20~30℃, 1000~2000rpm magnetic stirring digestion for 1~3h, filtration and washing until neutral, and drying at 60~80℃ for 2~6h; the ultrasonic vibration recrystallization purification pretreatment parameters are: soda ash to high-purity water mass ratio 1~2:1, adding 1~5mL of ferric chloride solution with a concentration of 1~3mg / mL, 60~80℃, 25~35KHz. The mixture is subjected to cyclic ultrasonication for 1-10 hours, filtered, concentrated, and crystallized, then dried at 100-110℃ for 2-6 hours. The carbon powder is conductive carbon black or graphite powder with a particle size of 50-200 nm. The high-temperature impurity removal pretreatment parameters are calcination at 400-600℃ for 1-2 hours under an argon atmosphere.

[0008] Further, in step 1②, the ultra-low temperature liquid nitrogen ball milling parameters are: ball-to-material mass ratio 1~3:1, grinding balls are 2~5mm zirconia balls, after immersion in liquid nitrogen, first grinding at 3~10Hz for 0.5~1h, then grinding at 10~30Hz for 1~2h to obtain 200~400 mesh mixed powder; in step 1③, the inert atmosphere is nitrogen or argon, the pressing pressure is 10~300MPa, and the tablet blank size is 20~30mm×20~30mm×5~10mm.

[0009] In step 1, the pre-sintering parameters of the atmosphere-protected electromagnetic rotary kiln are: argon flow rate 500~1000mL / min, heating rate 5~10℃ / min, pre-sintering temperature 650~800℃, holding time 2~4h, and rotary kiln speed 5~15r / min; the final sintering parameters of the atmosphere-protected vacuum sintering furnace are: vacuum degree 50~150Pa, heating rate 3~5℃ / min, final sintering temperature 850~1000℃, holding time 1~3h, and a hydrogen / argon mixture with a volume fraction of 5~10% is introduced for reduction and passivation in the later stage of sintering.

[0010] Further, in step (1)⑤, the low-temperature liquid nitrogen ball milling parameters are as follows: silicon carbide grinding ball diameter 5~10mm, ball-to-material ratio 6~12:1, ball milling at 180~300rpm for 1~2h to obtain 50~100 mesh powder; polar solvent is one of anhydrous ethanol, methanol, acetone or acetonitrile, ultrasonic cleaning frequency 35~55KHz for 1~2h; vacuum drying temperature 60~80℃ for 2~4h.

[0011] Further, in step two①, the molten sugar is at least one of glucose, sucrose, fructose or maltitol, with a melting temperature of 150~200℃, a stirring speed of 500~1000rpm, and a stirring time of 30~60min.

[0012] In step 2②, the segmented heating sintering carbonization parameters are as follows: First stage: vacuum degree 50~150Pa, heating to 200~300℃ at 5~8℃ / min, holding for 1~2h to remove moisture and low boiling point impurities; Second stage: maintaining vacuum degree, heating to 600~800℃ at 3~5℃ / min, holding for 2~4h to achieve sugar carbonization and carbonization; Third stage: natural cooling to room temperature.

[0013] Furthermore, the high-purity sodium chromite with zero hexavalent chromium obtained in step one has a purity of ≥99.8%, no hexavalent chromium residue, a hexagonal crystal structure, and a particle size of 0.5~20μm; the carbon coating layer on the surface of the high-conductivity product obtained in step two has a thickness of 3~10nm and an electronic conductivity of 10⁻²~10⁻¹ S / cm.

[0014] Another object of the present invention is to provide a highly conductive, high-purity sodium chromite with zero hexavalent chromium prepared according to the preparation method described above.

[0015] Another object of the present invention is to provide the application of the aforementioned high-conductivity, high-purity sodium chromite with zero hexavalent chromium in the preparation of cathode materials for sodium-ion batteries.

[0016] Compared with existing technologies, the present invention achieves the following significant technical effects through the synergistic effect of raw material pretreatment, two-step sintering purification and molten sugar carbonization modification.

[0017] First, this invention achieves ultra-high purity sodium chromite with zero hexavalent chromium residue. Through multi-stage pretreatment—nitric acid-high-pressure closed digestion of chromium oxide, ultrasonic recrystallization of soda ash, and high-temperature impurity removal of carbon powder—the introduction of heavy metals and impurities is eliminated at the source. Combined with the two-step process of "pre-sintering impurity removal + vacuum final sintering reduction" in the first sintering step, the highly toxic hexavalent chromium (Cr) is effectively suppressed. 6 The process generates sodium chromite (⁺) and ensures the complete reduction of trace amounts of hexavalent chromium. Testing shows that the sodium chromite prepared using this invention has a purity ≥99.8% and no detectable hexavalent chromium (ND), fully complying with stringent environmental standards such as EU RoHS, greatly expanding its safety applications in consumer electronics, medical, and food contact materials.

[0018] Secondly, it significantly improves the electronic conductivity and electrochemical performance of the material. Addressing the core weakness of poor intrinsic conductivity of sodium chromite, this invention innovatively introduces a molten sugar carbonization modification step. Utilizing the excellent fluidity of molten sugar at high temperatures, it achieves comprehensive and uniform coating of the sodium chromite particle surface; after three-stage vacuum sintering, the amorphous carbon layer formed by sugar carbonization bonds tightly with the matrix interface, constructing a continuous and stable three-dimensional conductive network. Tests show that the electronic conductivity of the obtained product is significantly improved compared to the traditional 10⁻⁻⁶. 5 ~10⁻ 6 The S / cm ratio has been increased to 10⁻²~10⁻¹ S / cm, an improvement of more than three orders of magnitude, which effectively reduces the battery polarization resistance, thereby enabling the assembled sodium-ion battery to exhibit excellent rate performance and cycle stability.

[0019] Third, the process is green and economical, and easily scaled up for industrial production. This invention uses inexpensive, readily available, and environmentally friendly sugars as a carbon source, replacing expensive graphene, carbon nanotubes, or complex organic complexing agents, significantly reducing raw material costs. Simultaneously, the entire preparation process (electromagnetic rotary kiln pre-sintering, vacuum sintering furnace final sintering, and carbon fixation) utilizes mature, commercially available equipment. Process parameters (temperature, pressure, atmosphere) are highly controllable, eliminating the need for complex chemical vapor deposition or precise atmosphere control, making it suitable for large-scale, continuous industrial production and possessing extremely high commercial potential.

[0020] Fourth, it exhibits excellent structural stability and interfacial compatibility. Because the molten sugar carbonization process is carried out in a mild vacuum environment, it avoids the damage to the sodium chromite lattice structure caused by high-temperature oxidation or strong acid corrosion, ensuring the material's excellent crystal integrity. The resulting carbon coating layer is uniform in thickness and robust, and is not prone to peeling or pulverization during long-term charge-discharge cycles, thus endowing the electrode material with excellent long-term cycle stability. Detailed Implementation

[0021] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0022] It should be noted that the terms "first," "second," etc., used in the specification and claims of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be used interchangeably where appropriate to understand the embodiments of the invention described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a product or device comprising a series of units is not necessarily limited to those explicitly listed, but may include other units not explicitly listed or inherent to such product or device.

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. Example 1 includes the following steps.

[0024] Step 1: Preparation of high-purity sodium chromite with zero hexavalent chromium: (1) Pretreatment of raw materials: Single crystal Cr2O3 with a particle size of 1~30μm was mixed with 10wt% nitric acid solution at a solid-liquid mass ratio of 1:15, and digested under high pressure at 8MPa, 25℃ and 1500rpm for 2h. After filtration and washing until neutral, it was dried at 70℃ for 4h. Anhydrous sodium carbonate and high-purity water were mixed at a mass ratio of 1.5:1, and 3mL of ferric chloride solution with a concentration of 2mg / mL was added. The mixture was subjected to cyclic ultrasonic treatment at 70℃ and 30KHz for 5h. After filtration, concentration and crystallization, it was dried at 105℃ for 4h. Conductive carbon black with a particle size of 100nm was calcined at 500℃ for 1.5h under an argon atmosphere to remove surface impurities. (2) Mixing: Pretreated chromium oxide, soda ash and conductive carbon black are mixed in a molar ratio of 1:1.05:0.03. Zirconia grinding balls with a diameter of 3 mm (ball-to-material mass ratio of 2:1) are added. After filling with liquid nitrogen, the mixture is first ground at a low frequency of 5 Hz for 0.8 h, and then ground at a high frequency of 20 Hz for 1.5 h. After the liquid nitrogen evaporates, a 300-mesh mixed powder is obtained. (3) Atmosphere-protected molding: Under an argon atmosphere, the mixed powder is pressed into a 25 mm × 25 mm × 8 mm tablet blank at a pressure of 150 MPa. (4) Two-step sintering: The pressed blank is placed in an atmosphere-protected electromagnetic rotary kiln with an argon flow rate of 800 mL / min, heated to 700℃ at 8℃ / min, held for 3h, and rotated at 10 r / min; then transferred to an atmosphere-protected vacuum sintering furnace, evacuated to 100 Pa, heated to 900℃ at 4℃ / min, held for 2h, and then a hydrogen / argon mixture with a volume fraction of 8% is introduced for reduction passivation. (5) Post-treatment: The final sintered product is mixed with silicon carbide grinding balls with a diameter of 8 mm at a ball-to-material mass ratio of 10:1, ball-milled at 250 rpm for 1.5h, then ultrasonically cleaned with anhydrous ethanol at a frequency of 45 kHz for 1.5h, and finally vacuum dried at 70℃ for 3h to obtain high-purity sodium chromite with zero hexavalent chromium.

[0025] The obtained product has a purity of 99.92%, no hexavalent chromium residue, a hexagonal crystal structure, and a particle size distribution of 5~15μm.

[0026] Step 2, Vacuum carbonization modification of molten sugar: (1) Molten sugar mixing: Heat sucrose to 180℃ to melt it, mix the high-purity sodium chromite obtained in step 1 with the molten sugar at a mass ratio of 100:8, stir at 800 rpm for 45 min to form a uniform coating slurry. (2) Vacuum carbonization sintering: Transfer the coating slurry to a vacuum sintering furnace under atmosphere protection, with the vacuum degree controlled at 100 Pa. First, raise the temperature to 250℃ at 6℃ / min and hold for 1.5 h; then raise the temperature to 700℃ at 4℃ / min and hold for 3 h; finally, cool naturally to room temperature to obtain high-conductivity, high-purity sodium chromite with zero hexavalent chromium.

[0027] Testing revealed that the carbon coating on the product surface is 5-7 nm thick, and the electronic conductivity is 3.2 × 10⁻² S / cm.

[0028] Electrochemical performance testing: The obtained material was mixed with acetylene black and polyvinylidene fluoride at a mass ratio of 85:5:10, N-methylpyrrolidone was added to form a slurry, which was then coated onto aluminum foil, dried, and pressed into a positive electrode sheet. Using metallic sodium as the negative electrode, glass fiber as the separator, and 1 mol / L NaPF6 (PC:EC=1:1) as the electrolyte, CR2032 coin cells were assembled in an argon glove box. Within a voltage range of 2.0–3.6 V, the first-cycle discharge specific capacity at 0.1C was 112.8 mAh / g; the discharge specific capacity at a high rate of 10C was 98.5 mAh / g; and the capacity retention after 1000 cycles at 1C was 97.2%.

[0029] Furthermore, embodiments a, b, and c are also provided, wherein embodiments a and b are the technical solutions corresponding to the front and rear endpoint values ​​of the specific parameters with numerical ranges described in the technical solution section of this specification, and embodiment c is the technical solution corresponding to the midpoint value of the specific parameters with numerical ranges described in the technical solution section of this specification, which will not be elaborated further. Example 2 includes the following steps.

[0030] Step 1, Preparation of high-purity sodium chromite with zero hexavalent chromium: (1) Raw material pretreatment: 1~30μm single crystal Cr2O3 and 20wt% nitric acid solution are mixed at a solid-liquid mass ratio of 1:10, digested at 10MPa, 30℃ and 2000rpm for 1h, washed and dried; anhydrous sodium carbonate and high-purity water are mixed at a mass ratio of 2:1, 5mL of ferric chloride solution with a concentration of 3mg / mL is added, and the mixture is cyclically sonicated at 80℃ and 35KHz for 10h, concentrated, crystallized and dried; 50nm graphite powder is calcined at 600℃ for 1h under argon atmosphere. (2) Mixing ratio: Chromium oxide, soda ash and graphite powder are mixed at a molar ratio of 1:1.2:0.05, 5mm zirconia balls (ball-to-material ratio 3:1), ball milled in liquid nitrogen (10Hz×0.5h+30Hz×1h) to obtain 400 mesh powder. (3) Atmosphere protection molding: Nitrogen atmosphere, 300MPa pressing into 30mm×30mm×10mm billets. (4) Two-step sintering: Electromagnetic rotary kiln argon flow rate 1000mL / min, heating to 800℃ at 10℃ / min and holding for 2h (rotation speed 15r / min); vacuum sintering furnace 50Pa, heating to 1000℃ at 5℃ / min and holding for 1h, followed by the introduction of 10% hydrogen / argon mixed gas. (5) Post-treatment: 10mm silicon carbide balls (ball-to-material ratio 12:1) ball milled at 300rpm for 1h, ultrasonicated with methanol at 55KHz for 1h, and vacuum dried at 80℃ for 2h.

[0031] The resulting product has a purity of 99.88%, no hexavalent chromium residue, and a particle size of 8~20μm.

[0032] Step 2, Vacuum carbonization modification of molten sugar: (1) Molten sugar mixing: Glucose is melted at 150℃, and high-purity sodium chromite is mixed with molten sugar at a ratio of 100:15 and stirred at 1000rpm for 30min. (2) Vacuum carbonization sintering: Vacuum degree 50Pa, temperature is increased to 300℃ at 8℃ / min and held for 1h, temperature is increased to 800℃ at 5℃ / min and held for 2h, and then cooled naturally.

[0033] Tested results show that the carbon coating thickness is 7-10 nm, and the electronic conductivity is 8.5 × 10⁻² S / cm. The first discharge capacity at 0.1C is 110.5 mAh / g, at 10C it is 96.3 mAh / g, and the capacity retention rate after 1000 cycles at 1C is 96.8%. Example 3 includes the following steps.

[0034] Step 1, Preparation of high-purity sodium chromite with zero hexavalent chromium: (1) Pretreatment of raw materials: 1~30μm single crystal Cr2O3 and 5wt% nitric acid solution are mixed at 1:20, digested at 5MPa, 20℃, and 1000rpm for 3h, washed and dried; anhydrous sodium carbonate and high-purity water are mixed at 1:1, 1mL of ferric chloride solution with a concentration of 1mg / mL is added, and ultrasonically circulated at 60℃ and 25KHz for 1h, concentrated, crystallized and dried; 200nm conductive carbon black is calcined at 400℃ for 2h under argon atmosphere. (2) Mixing ratio: Chromium oxide, soda ash and conductive carbon black are mixed at a molar ratio of 1:1.02:0.01, 2mm zirconium oxide balls (ball-to-material ratio 1:1), ball milled in liquid nitrogen (3Hz×1h+10Hz×2h) to obtain 200 mesh powder. (3) Atmosphere protection molding: Argon atmosphere, 10MPa is used to press into 20mm×20mm×5mm blanks. (4) Two-step sintering: In the electromagnetic rotary kiln, the argon flow rate is 500 mL / min, the temperature is raised to 650℃ at 5℃ / min and held for 4 hours (rotation speed 5 r / min); in the vacuum sintering furnace, the temperature is raised to 850℃ at 150 Pa at 3℃ / min and held for 3 hours, and a 5% hydrogen / argon mixture is introduced in the later stage. (5) Post-treatment: 5 mm silicon carbide balls (ball-to-material ratio 6:1) are ball-milled at 180 rpm for 2 hours, ultrasonicated with acetone at 35 kHz for 2 hours, and vacuum dried at 60℃ for 4 hours.

[0035] The resulting product has a purity of 99.85%, no hexavalent chromium residue, and a particle size of 0.5~5μm.

[0036] Step 2, Vacuum carbonization modification of molten sugar: (1) Molten sugar mixing: maltitol is melted at 200℃, and high-purity sodium chromite is mixed with molten sugar at a ratio of 100:5 and stirred at 500rpm for 60min. (2) Vacuum carbonization sintering: vacuum degree 150Pa, temperature is increased to 200℃ at 5℃ / min and held for 2h, temperature is increased to 600℃ at 3℃ / min and held for 4h, and then cooled naturally.

[0037] The carbon coating thickness is 3~5nm, and the electronic conductivity is 1.8×10⁻² S / cm. The first discharge specific capacity at 0.1C is 109.2mAh / g, at 10C it is 94.7mAh / g, and the capacity retention rate is 95.9% after 1000 cycles at 1C. Comparative Example 1 (without carbon fixation modification)

[0038] High-purity sodium chromite with zero hexavalent chromium was prepared only according to the first step of Example 1, without the second step of molten sugar carbonization modification. The resulting product had an electronic conductivity of 4.5 × 10⁻⁻⁻⁴. 5 S / cm. The first discharge specific capacity at 0.1C is 108.3mAh / g, while the discharge specific capacity at 10C rate is only 62.8mAh / g. After 1000 cycles at 1C, the capacity retention rate is 83.5%.

[0039] The results showed that uncoated sodium chromite had poor conductivity and significantly deteriorated rate performance and cycle stability. Comparative Example 2 (using ordinary sucrose powder mixed and sintered)

[0040] Following the first step of Example 1 to prepare high-purity sodium chromite with zero hexavalent chromium, the second step was modified as follows: high-purity sodium chromite was directly mixed with sucrose powder at a mass ratio of 100:8 without melting, and then vacuum sintered under the same conditions. The resulting product had an uneven carbon coating layer on its surface, with localized agglomeration, and an electronic conductivity of 8.2 × 10⁻³ S / cm. The first-cycle discharge specific capacity at 0.1C was 107.6 mAh / g, the discharge specific capacity at 10C was 75.3 mAh / g, and the capacity retention rate after 1000 cycles at 1C was 88.7%.

[0041] The results show that unmelted sucrose cannot form a uniform and dense carbon coating layer, resulting in limited improvement in conductivity and electrochemical performance. Comparative Example 3 (sintering only once, without two-step sintering)

[0042] Following the raw material ratio and pretreatment method of Example 1, but combining pre-sintering and final sintering into a single sintering process: the tablet blank was directly heated to 900℃ at 4℃ / min in a vacuum sintering furnace and held for 5 hours, without electromagnetic rotary kiln pre-sintering and hydrogen reduction passivation. The resulting product showed hexavalent chromium residue (approximately 0.15wt%), a purity of only 97.2%, and uneven particle size distribution (1~35μm). After molten sugar carbonization modification under the same conditions, the electronic conductivity was 2.1×10⁻² S / cm, but the first-cycle discharge specific capacity at 0.1C was only 98.4mAh / g, and the capacity retention dropped to 82.5% after 500 cycles at 1C.

[0043] The results show that the lack of pre-sintering and reduction passivation steps leads to hexavalent chromium residue and incomplete crystal structure, which seriously affects electrochemical performance.

[0044] The performance comparison table above is as follows.

[0045] Example 1 99.92 none 5-7 <![CDATA[3.2×10 - ²]]> 112.8 98.5 97.8 Example 2 99.88 none 7-10 <![CDATA[8.5×10 - ²]]> 110.5 96.3 96.8 Example 3 99.85 none 3-5 <![CDATA[1.8×10 - ²]]> 109.2 94.7 95.9 Comparative Example 1 99.92 none — <![CDATA[4.5×10 -5 ]]> 108.3 62.8 83.5 Comparative Example 2 99.90 none Uneven <![CDATA[8.2×10 -3 ]]> 107.6 75.3 88.7 Comparative Example 3 97.20 have 4-8 <![CDATA[2.1×10 - ²]]> 98.4 — 82.5 (500 laps)

[0046] Analysis of experimental results of this invention: Step 1 effect verification: Hexavalent chromium: alkaline extract + DCP method detection, Cr 6 ⁺ Not detected (<10 ppm); Purity: XRF main component ≥99.92%, total impurities <0.08%; Phase: XRD shows a single sodium chromite phase, without Cr2O3 / Na2CrO4 impurity peaks, and the cell parameters are consistent with standard NaCrO2. Pre-sintering removes volatile impurities, and final sintering inhibits the formation of hexavalent chromium. These two steps work synergistically to achieve the dual goals of zero hexavalent chromium and high purity. Step 2 Effect Verification: Carbon Layer Morphology: TEM showed that the carbon coating thickness of the product was 3~10nm, with electronic conductivity of 1.8×10⁻²~8.5×10⁻² S / cm and no pores at the interface; Purity Retention: The purity of the carbonized product was ≥99.8% (the carbon layer mainly contributed by C, excluding metal impurities); Cycling Stability: The discharge specific capacity at 0.1C rate was 109.2~112.8mAh / g in the first cycle; the discharge specific capacity at 10C high rate was 94.7~98.5mAh / g; after 1000 cycles at 1C rate, the capacity retention rate was 95.9~97.2%. Molten sugar achieves full-surface wetting and coating of sodium chromite particles, and vacuum segmented carbon fixation avoids oxidation and impurity introduction, improving conductivity by two orders of magnitude while maintaining high purity. Example 4

[0047] By leveraging the "synergistic effect" between pretreatment steps and introducing "controllable oxygen vacancies" through a carbon coating process, both purity and intrinsic conductivity are improved. This embodiment includes the following steps.

[0048] Pretreatment (synergistic purification): Chromium oxide treatment: A two-step acid treatment method was adopted. First, Cr2O3 was stirred at 80°C for 1 hour with a 5% oxalic acid solution (solid-liquid ratio 1:10) to remove surface alkaline impurities and some iron. After filtration, without washing, it was directly transferred to a 10% nitric acid solution for a second digestion under the high pressure (8MPa) conditions of Example 1. (Oxalic acid treatment forms a soluble complex, opening a channel for the high-pressure nitric acid to completely remove deep silicon and iron impurities. The two steps work synergistically, and the purification efficiency is far greater than that of a single step); Soda ash treatment: After the ultrasonic purification in Example 1, a low-temperature recrystallization step was added: The purified Na2CO3 saturated solution was slowly cooled to -5°C at a rate of 0.5°C / min and held for 2 hours to obtain needle-shaped high-purity crystals. (Low-temperature recrystallization can further eliminate trace amounts of organic impurities and eutectic ions to obtain ultrapure raw materials); Proportioning and sintering (in-situ introduction of defects): 0.5 mol% ammonium chloride (NH4Cl) was additionally introduced into the mixed powder D as a pore-forming agent and chlorine source. Everything else is the same as in Example 1.

[0049] During the final sintering holding stage (900℃), the atmosphere was switched from 8% H2 / Ar to an argon mixture containing 2% chlorine (Cl2) for 15 minutes, and then switched back to H2 / Ar reduction passivation. (Trace amounts of Cl2 can selectively etch oxygen in the lattice at high temperatures, introducing a controllable concentration of oxygen vacancies in situ into the NaCrO2 subsurface layer. These oxygen vacancies can act as electron donors, significantly improving the intrinsic electronic conductivity of the material); Gradient carbon coating (interface optimization): Instead of using a single sugar, a composite carbon source of glucose and polyvinylpyrrolidone (PVP) in a mass ratio of 9:1 was used.

[0050] Three-stage sintering parameter fine-tuning: After heating to 750℃ in the second stage, hold at that temperature for 1 hour, then slowly cool down to 650℃ at a rate of 1℃ / min, and hold for another 2 hours. (The addition of PVP makes the carbon layer more elastic and more firmly bonded to the particles; the slow cooling process results in a higher degree of graphitization of the carbon layer and forms a gradient structure from the particle surface to the interior of the carbon layer, optimizing the interfacial contact resistance).

[0051] Performance testing and in-depth analysis: Purity: >99.97%, total impurities <100 ppm. Hexavalent chromium: Not detected (<0.5 ppm). Conductivity: The conductivity of the compacted powder jumps to 58.7 S / cm.

[0052] Mechanism verification: XPS / EPR test: confirmed that the product contains a higher concentration of oxygen vacancies than usual.

[0053] In-situ electrochemical impedance spectroscopy: shows that the charge transfer resistance is reduced by approximately 60% compared to the product of Example 1.

[0054] Control experiment: A parallel experiment was set up with no NH4Cl introduced and no Cl2 passed through, all other things being exactly the same. The conductivity of the product obtained was 18.5 S / cm. This comparison directly proves the huge contribution of "in-situ introduction of oxygen vacancies" to conductivity (more than 3 times increase), rather than the effect of simple carbon coating. Example 5

[0055] Sodium chromite composite materials with a "core-shell-porous" secondary structure were prepared through a unique molding and sintering process, breaking through the physical limits of conductive networks in powder materials. This embodiment includes the following steps.

[0056] Precursor preparation: Same as steps one and two in Example 1, to obtain mixed powder D.

[0057] Cryogenic molding: Powder D is mixed with 3 wt% of an aqueous solution of polyacrylic acid (dispersant) at a solid-liquid ratio of 1:0.8 to prepare a high solid content slurry.

[0058] The slurry is injected into a mold with a regular array of columnar protrusions and then quickly immersed in liquid nitrogen for freezing and shaping.

[0059] Ice crystals are removed by sublimation in a vacuum freeze dryer to obtain a green preform with a regular vertical channel structure. (Freeze forming constructs pre-designed macroscopic ion / electron transport channels in the preform).

[0060] Segmented Atmosphere Sintering: Pre-sintering: In flowing N2, the temperature is increased to 700℃ at 5℃ / min and held for 2 hours. This ensures the green body acquires sufficient strength. Final sintering: Transferred to a vacuum furnace and evacuated. The "oscillating heating" method is used: 850℃ held for 30 minutes → decreased to 800℃ and held for 30 minutes → increased to 880℃ and held for 1 hour. (Oscillating heating helps with uniform grain growth and defect repair, resulting in a more complete crystal structure). Subsequent reduction is the same as in Example 1, using H2 / Ar.

[0061] Chemical vapor deposition (CVD) carbon coating: The sintered porous preform is placed in a CVD furnace, using ethylene (C2H4) as the carbon source and argon as the carrier gas. At 650℃, by controlling the C2H4 flow rate and deposition time (30 min), a graphene-like carbon layer approximately 5 nm thick is uniformly deposited on the inner and outer surfaces of the NaCrO2 framework. (CVD allows for precise control of carbon layer thickness and crystallinity, and results in extremely high bonding strength with the substrate, forming a perfect conductive network).

[0062] Performance testing and in-depth analysis: Structural characterization: SEM showed that the material has three-dimensional interconnected macropores (~10μm) and micropores in the framework itself, forming a "double continuous" structure.

[0063] Electrical conductivity: The volume conductivity was tested directly using the blank as an electrode, reaching 210 S / cm (far exceeding that of powder compaction).

[0064] Functional validation: When this material was directly used as an anode material for lithium-ion batteries, it maintained a capacity retention of 92% after 500 cycles at 1C, far superior to electrodes made from conventional powders (typically <85%). This application data strongly demonstrates the superiority of this microstructure design in real electrochemical environments.

[0065] Control experiment: Electrodes made from the same material powder using the traditional coating method showed that the overall surface conductivity of the electrode sheet was only 1 / 5 of that of the product in this embodiment. This proves that the "structural design" itself brings about a qualitative change in performance. Example 6

[0066] This embodiment pushes multiple parameters in the solution to their limits or uses unconventional combinations to address specific high-end application requirements (such as fillers for ultra-high frequency electronic components) and verifies their stability. The steps included in this embodiment are as follows.

[0067] Extreme purification of raw materials: Chromium oxide: After high-pressure nitric acid digestion, microwave-assisted hydrothermal cleaning (200℃, 2MPa, 30min) is added, using ultrapure water with resistivity >18.2 MΩ·cm.

[0068] Soda ash: After ultrasonic purification, it is further purified by zone melting to reduce the total amount of impurities to below ppm.

[0069] Carbon-free sintering: Carbon powder (C) is intentionally omitted, and only ultra-high purity chromium oxide and soda ash are used in a molar ratio of 1:1.05. (This explores the possibility of preparing zero-hexavalent chromium products solely by precisely controlling the oxygen partial pressure without external carbon reducing agents, completely eliminating the risk of carbon residue).

[0070] Ultra-high pressure oxygen partial control sintering: using a sintering furnace with precisely adjustable oxygen partial pressure (pO2).

[0071] The green body was heated to 950℃ in ultra-high purity Ar (O2<0.1 ppm) at a rate of 10℃ / min.

[0072] During the heat preservation stage, a trace amount of O2 is precisely introduced into Ar through a gas mixer to dynamically maintain the pO2 in the furnace at 10⁻¹. 5 The process was carried out for 2 hours within an extremely narrow window of ~10⁻¹³ atm. (This pO2 window is located precisely within the thermodynamic range where Cr₂O₃ is stable while Cr(VI) compounds are unstable, thus fundamentally preventing the formation of hexavalent chromium thermodynamically.) The process was then programmed to cool.

[0073] Atomic layer deposition (ALD) interface modification: The sintered product is subjected to ALD cycle of trimethylaluminum (TMA) and ozone (O3) at 200℃ to deposit Al2O3 for 3 cycles, forming a sub-monolayer cover.

[0074] Subsequently, the molten sucrose coating and carbonization were carried out according to the method of Example 1. (Innovation: The ultrathin Al2O3 layer, as an interface layer, can greatly enhance the chemical bonding between the carbon layer and the NaCrO2 matrix and prevent interface degradation during long-term use).

[0075] Performance Testing and In-Depth Analysis: Limiting Purity: ICP-MS showed total metallic impurities <10 ppm, meeting electronic-grade material standards. Hexavalent Chromium: Detected using more sensitive ion chromatography-mass spectrometry (IC-MS), it was undetectable (detection limit <0.1 ppb). Conductivity and Stability: Conductivity is 12.5 S / cm. Key Innovation: High-Frequency Performance: In the 1-10 GHz frequency range, its imaginary part of the composite dielectric constant (characterizing loss) is extremely low and exceptionally stable, with a quality factor (Q value) more than 5 times that of conventional products. Accelerated Aging Test: After 1000 hours of storage in a high-temperature and high-humidity environment of 85℃ / 85%RH, conductivity decay was <2%, and hexavalent chromium remained undetectable. This demonstrates that the method achieves extreme chemical stability and reliability.

[0076] Comparison and Value: The combination of "carbon-free + ultra-precise oxygen partial pressure control + ALD interface engineering" in this embodiment provides a brand-new solution for military, aerospace or medical electronics fields that require extreme purity, zero harmful substance leakage and ultra-high frequency stability. This is something that traditional methods cannot achieve, reflecting the cutting-edge nature and high level of creativity of the solution.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing high-conductivity, high-purity sodium chromite with zero hexavalent chromium, characterized in that, include: Step 1, Preparation of high-purity sodium chromite with hexavalent chromium: ① Raw material pretreatment: Chromium oxide is pretreated by nitric acid-high pressure closed digestion, soda ash is pretreated by ultrasonic vibration recrystallization purification, and carbon powder is pretreated by high temperature impurity removal; ② Mixing: The pretreated chromium oxide, soda ash, and carbon powder are mixed in a molar ratio of 1:(1.02~1.2):(0.01~0.05), and then pulverized by ultra-low temperature liquid nitrogen ball milling to obtain a uniform mixed powder; ③ Atmosphere-protected molding: The mixed powder is pressed into a tablet blank under an inert atmosphere to obtain a tablet blank; ④ Two-step sintering: In the first step, the tablet blank is placed in an atmosphere-protected electromagnetic rotary kiln for pre-sintering to achieve preliminary solid-phase reaction of the raw materials and removal of impurities. In the second step, it is transferred to an atmosphere-protected vacuum sintering furnace for final sintering to ensure the presence of hexavalent chromium; ⑤ Post-treatment: The final sintered product is ball-milled by low temperature liquid nitrogen, ultrasonically cleaned with polar solvent, and vacuum dried to obtain high-purity sodium chromite with hexavalent chromium; Step 2, carbon fixation modification to improve conductivity: ① Molten sugar mixing: Mix the high-purity sodium hexavalent chromium chromite obtained in step (1) with molten sugar at a mass ratio of 100:(5~15) and stir until a uniform coating slurry is formed; ② Vacuum carbon fixation sintering: Transfer the coating slurry to a vacuum sintering furnace under atmosphere protection and sinter it in three stages. The first stage is used to remove moisture and low-boiling-point impurities, the second stage is used to achieve carbonization and carbon fixation of sugar, and the third stage is cooled to obtain high-conductivity high-purity sodium hexavalent chromium chromite.

2. The method for preparing high-conductivity, high-purity sodium chromite with zero hexavalent chromium according to claim 1, characterized in that, In step 1①, the chromium oxide is large-size single-crystal Cr2O3 with a particle size of 1~30μm; the nitric acid-high-pressure closed digestion pretreatment parameters are: chromium oxide to 5~20wt% nitric acid solution solid-liquid mass ratio 1:10~20, 5~10MPa, 20~30℃, 1000~2000rpm magnetic stirring digestion for 1~3h, followed by filtration and washing until neutral, and drying at 60~80℃ for 2~6h; the ultrasonic vibration recrystallization purification pretreatment parameters are: soda ash to high-purity water mass ratio 1~2:1, addition of 1~5mL of ferric chloride solution with a concentration of 1~3mg / mL, 60~80℃, 25~35KHz. The mixture is subjected to cyclic ultrasonication for 1-10 hours, filtered, concentrated, and crystallized, then dried at 100-110℃ for 2-6 hours. The carbon powder is conductive carbon black or graphite powder with a particle size of 50-200 nm. The high-temperature impurity removal pretreatment parameters are calcination at 400-600℃ for 1-2 hours under an argon atmosphere.

3. The method for preparing high-conductivity, high-purity sodium chromite with zero hexavalent chromium according to claim 1, characterized in that, In step (1) ②, the ultra-low temperature liquid nitrogen ball milling parameters are: ball-to-material mass ratio 1~3:1, grinding balls are 2~5mm zirconia balls, after immersion in liquid nitrogen, first grind at 3~10Hz for 0.5~1h, then grind at 10~30Hz for 1~2h to obtain 200~400 mesh mixed powder; in step (1) ③, the inert atmosphere is nitrogen or argon, the pressing pressure is 10~300MPa, and the tablet blank size is 20~30mm×20~30mm×5~10mm.

4. The method for preparing high-conductivity, high-purity sodium chromite with zero hexavalent chromium according to claim 1, characterized in that, In step 1, the pre-sintering parameters of the atmosphere-protected electromagnetic rotary kiln are: argon flow rate 500~1000mL / min, heating rate 5~10℃ / min, pre-sintering temperature 650~800℃, holding time 2~4h, and rotary kiln speed 5~15r / min; the final sintering parameters of the atmosphere-protected vacuum sintering furnace are: vacuum degree 50~150Pa, heating rate 3~5℃ / min, final sintering temperature 850~1000℃, holding time 1~3h, and a hydrogen / argon mixture with a volume fraction of 5~10% is introduced for reduction and passivation in the later stage of sintering.

5. The method for preparing high-conductivity, high-purity sodium chromite with zero hexavalent chromium according to claim 1, characterized in that, In step 1, the parameters for the cryogenic liquid nitrogen ball milling are as follows: silicon carbide grinding ball diameter 5~10mm, ball-to-material ratio 6~12:1, ball milling at 180~300rpm for 1~2h to obtain 50~100 mesh powder; the polar solvent is one of anhydrous ethanol, methanol, acetone or acetonitrile; ultrasonic cleaning frequency 35~55KHz for 1~2h; vacuum drying temperature 60~80℃ for 2~4h.

6. The preparation method according to claim 1, characterized in that, In step 2①, the molten sugar is at least one of glucose, sucrose, fructose or maltitol, with a melting temperature of 150~200℃, a stirring speed of 500~1000rpm, and a stirring time of 30~60min.

7. The preparation method according to claim 1, characterized in that, In step 2②, the segmented heating sintering and carbonization parameters are as follows: First stage: vacuum degree 50~150Pa, heating to 200~300℃ at 5~8℃ / min, holding for 1~2h to remove moisture and low boiling point impurities; Second stage: Maintain vacuum, heat to 600-800℃ at 3-5℃ / min, hold for 2-4 hours to achieve carbonization and carbon fixation of sugar; Third stage: Cool naturally to room temperature.

8. The preparation method according to claim 1, characterized in that, The high-purity sodium chromite with zero hexavalent chromium obtained in step one has a purity of ≥99.8%, no hexavalent chromium residue, a hexagonal crystal structure, and a particle size of 0.5~20μm; the carbon coating layer on the surface of the high-conductivity product obtained in step (2) has a thickness of 3~10nm and an electronic conductivity of 10⁻²~10⁻¹ S / cm.

9. Highly conductive, high-purity sodium chromite with zero hexavalent chromium prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the high-conductivity, high-purity sodium chromite with zero hexavalent chromium as described in claim 9 in the preparation of cathode materials for sodium-ion batteries.